Well, i will leave here the best locations where you should death abuse in each level, and if needed, i'm going to explain the reasons.

Why death abuse?

You can learn this from experience: When you die after hitting a checkpoint box, then you're unable to see the scene where all the boxes you missed on the level falls into Crash's head (Except for NTSC-J, you can't skip the scene, but just 1 box falls into Crash's head). A single death after hitting a checkpoint can save several seconds for you in each level, that's the Death Abuse. You can also take more advantage of such death, because everytime Crash dies, the global cycles in the game resets, so you can use the death for cycle manipulation as well. So the places where you death abuse are also important, and here follows a list containing where you should Death Abuse:

N. Sanity Beach: There's just 1 checkpoint here. Death abuse on the first hole you see.

Jungle Rollers: Also has just 1 checkpoint. Hit the checkpoint, then backtrack a bit and die on the first hole.

The Great Gate: Just 1 checkpoint as well. Death abuse on the subsequent gap.

Boulders: Also just 1 checkpoint. Death abuse on the hole.

Upstream: There's 2 checkpoints. Death abuse on the first one, fall into the waterfall. (Faster death here than on the 2nd checkpoint)

Rolling Stones: There's 2 checkpoints. Death abuse on the first one, fall into the hole. (From this death abuse you can keep more masks through the level for damage boosts.)

Hog Wild: You don't Death Abuse here, because getting all boxes but 1 is faster than Death Abusing.

Native Fortress: There's 5 checkpoints. Death abuse on the 5th one, just fall into the gap. (You get the fastest cycles in the end.)

Up The Creek: There's 2 checkpoints. Death abuse on the 1st one, just fall into the water. Allows you to get better cycles in The Lost City. (If you were to be crazy enough to do all the double jumps in here, death abuse in the 2nd one).

The Lost City: You don't Death Abuse here. The Lost City has the green gem when you collect all the boxes, and you need the Green Gem to unlock a path in Castle Machinery where you skip the entire level directly to the warp platform.

Temple Ruins: There's 3 checkpoints here. Death abuse on the 1st one, just fall off the ledge.

Road to Nowhere: There's 4 checkpoints. Death abuse on the 4th one, just fall into the subsequent gap. (doesn't matters much where you death abuse here, but that's the place where i usually death abuse.)

Boulder Dash: There's 2 checkpoints here. Death abuse on the first one, just fall into the subsequent hole.(The 2nd checkpoint has only slow death possibilities)

Sunset Vista: The level with the highest number of checkpoints, 7 in the total. Death abuse on the 5th one, just fall off the ledge (down right diagonal). (The respawn will set Crash on the perfect position for the Out of Bounds, just walk straightly). (If you are doing the Game Over abuse strat, only death abuse after finishing the Tawna bonus).

Heavy Machinery: Has just 1 checkpoint. Just fall off the ledge.

Cortex Power: Has 2 checkpoints. Death abuse on the 2nd one. (You can have a pretty fast death abuse there)

Generator Room: Has 2 checkpoints. Death abuse on the 1st one. Just fall off the ledge. (I recommend taking the subsequent mask after this death abuse, and keeping it till the Red hot pipe, so you can damage boost on it. Death abusing on the 2nd checpoint makes you lose this mask).

Toxic Waste: Has 2 checkpoints as well. Death abuse on the 2nd one, just fall into the acid. (That way, the mask you get at the start lasts longer, for safety.)

The High Road: Has 4 checkpoints. Death abuse on the 4th one, just fall into the gap behind Crash. (This setups good initial cycles in Slippery Climb). (Death abusing on the 3rd one gives you slightly better cycles for Slippery, saving around a second, but it's trickier).

Slippery Climb: Has just 1 checkpoint. Death abuse on the spikes under the subsequent moving platform. (There's no holes or voids around to give you a faster death animation, that's the only option.)

Lights Out: There's 2 checkpoints. Death abuse on the first one, just fall off the ledge. (This death setups the fastest cycles for this level.)

Jaws of Darkness: Has 2 checkpoints as well. Death abuse on the 2nd one. (This death gives you good subsequent pillar cycles.)

Castle Machinery: If you got the Green Gem from Lost City, you can step on it and walk directly to the warp platform (you're unable to death abuse on this path). That saves a lot of time, even if you don't death abuse.

The Lab: Has just 1 checkpoint. Just fall off the ledge.

I will disponibilize a video containing the Death Abuse Locations in the game: